1. Field of the Invention
The inventions relate generally to optical telecommunications and, more particularly, to electro-optical modulators.
2. Discussion of the Related Art
One goal of telecommunications research is to increase transmission rates for digital data. Presently, optical transmitters transmit digital data at rates that are below desired values. The transmission rate of an optical transmitter is, in part, limited by the frequency at which a modulator is able to encode data onto a continuous optical carrier wave.
Typically, modulators amplitude-encode data onto the optical carrier wave using microwave or millimeter control waves. The control waves electro-optically modulate refractive indexes in optical waveguides of the modulator. The optical waveguides form arms of an optical interferometer. By modulating refractive indexes, the control waves modulate propagation times in the arms of the interferometer and thus, intensity distributions produced by the interferometer. From the interferometer, the pattern is sent to an output optical waveguide whose coupling depends on the received intensity distribution. Thus, the optical wave transmitted by the output waveguide, i.e., the modulator""s output optical wave, is amplitude-modulated by the control waves that control the form of the interference pattern.
For optimal modulation, velocities of the control and carrier waves should be matched in the electro-optical modulator. Otherwise, wavefronts of the control wave corresponding to different data periods will modulate the same portion of the optical carrier wave. The accuracy of the match between velocities of control and optical carrier waves provides an upper limit on the operating frequency of an electro-optical modulator.
In optical modulators, mismatches between control and optical carrier wave velocities occur for several reasons. First, optical wavelengths are typically between about 1.3 microns and about 1.7 microns and control wave wavelengths are typically in the centimeter to submillimeter. Since refractive indexes of the modulator""s dielectrics depend on wavelength, this wavelength dependence tends to produce a velocity mismatch between the optical carrier and control waves. Second, control waves include fringe field components, i.e., fields in air or vacuum. Since propagation velocities are higher in air and vacuum, fringe field components tend to make control-wave velocities higher than those of optical carrier waves, which propagate in condensed dielectrics of the electro-optical modulator.
In one aspect, the inventions feature electro-optical modulators that reduce mismatches between control and optical carrier-wave velocities by using dielectrics that compensate for the speeding up that fringe fields tend to produce in control waves Such electro-optical modulators include an optical waveguide for carrying an optical carrier wave and a control waveguide for carrying a control wave. The optical waveguide includes a cladding layer and a core. The two waveguides are collinear and the refractive index of the optical waveguide responds to electric fields generated by the control wave in the interaction region. The refractive index of the cladding layer at the control wave""s wavelength is higher than the refractive index of the core at the optical carrier wave""s wavelength.
In another aspect, the inventions feature electro-optical modulators in which dielectric portions have refractive index contrasts, at control-wave wavelengths, that increase electric field intensities produced by the control waves in the modulators"" optical waveguides. Increasing electric field intensities in the modulator""s optical waveguides improves couplings between the control and optical carrier waves.
One such optical modulator includes an optical waveguide for carrying an optical carrier wave and a control waveguide for carrying a control wave. The two waveguides are collinear. The optical waveguide includes a cladding layer and a core and has a refractive index that is responsive to electric fields in an interaction region. The refractive index of the core is lower than the refractive index of adjacent portions of the cladding layer at the wavelength of the control wave.
In another aspect, an embodiment of an optical modulator includes an interferometer having two optical waveguides with associated cores, a pair of electrodes extending parallel to one of the cores, and a cladding disposed between the one of the cores and the electrodes. The one of the cores has a refractive index that is responsive to applied electric fields. The refractive index of the one of the cores at a wavelength between about 1.3 microns and about 1.7 microns is smaller than the refractive index of the cladding at a microwave""s wavelength, a millimeter wave""s wavelength, or a submillimeter wave""s wavelength. At a microwave""s wavelength, a millimeter wave""s wavelength, or submillimeter wave""s wavelength, the ratio of the refractive index of the one of the cores to the refractive index of the cladding may be less than one.